Conventional punching consists of hitting a workpiece between a punch and a die, in such a way that the punch pierces the workpiece, and enters the die, creating a hole in said workpiece. This process is an example of shearing processes, where the workpiece is deformed until the ultimate strength is overcome, and the affected section breaks.
If the punch is performed in a sheet workpiece which is too thick, the punching operation produces a very poor quality result: when the punch enters the workpiece, a first zone with a burnished cut is created, but this regular cut only lasts for approximately the first third of the sheet thickness. Due to growing shear stresses, the rest of the inner shape of the hole is quite irregular, with a pseudo conical shape, the diameter of the hole growing wider as the punch goes deeper, further producing a burr at the end of the cut and also producing microscopic cracks. This scenario is not adequate for workpieces which are to be used in responsibility structures, as, besides the bad tolerances of the hole shape, these microscopic cracks could cause the failure of the workpiece and the collapse of the structure when it is subjected to cyclic stresses.
Drilling is preferred for those cases, as it produces a much more regular finishing, with better dimensional tolerances and without said cracks. Drilling is an example of chip removing processes, where a tool is used to locally cut a thin layer of the material thanks to a fast rotatory movement. However, drilling is much slower and more expensive than punching, since the drill advance direction is not the same as the drill cut direction (which is rotative). A punching procedure that produced a good surface finishing, without the conical shape and without the microscopic cracks would be more advantageous than drilling, as it would further be cheaper and faster.